This invention relates to web supporting apparatus and is more particularly concerned with a web supporting and guiding device for a web drying apparatus whereby a lengthwise moving web is floatingly supported and maintained in a straight path (i.e. centered with respect to the longitudinal centerline of the web) throughout the length of the web drying apparatus.
In drying a moving web of paper, film, or the like, it is desirable that the web be contactlessly supported during the drying operation, to avoid damage to the web itself or to an ink or coating on the web. One common arrangement for contactlessly supporting a web comprises upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Air issuing from the lower set of air bars floatingly supports the web, and air issuing from the upper set of air bars steadies the web to maintain it substantially straight and at a substantially constant distance from the air bars of both sets. The air blown from both sets of air bars is usually heated to expedite web drying, and typically the air bar array is inside an enclosure which is maintained at a slightly subatmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web.
Web dryers are used in many printing and graphics applications, such as the processing of photographic film, web offset printing, and other types of printing. In some applications, such as the processing of photographic film, web dryers having very long length, often as long as 150 feet, are commonly used. The greater the ratio of the length of the dryer to the width of the web (hereinafter referred to as the "length to width ratio"), the more susceptible the web is to minor forces that can cause the web to weave (move back and forth in a lateral direction) or shift (move laterally from the centerline and remain there). Also, some webs, such as thin plastic films, naturally take on a "banana-shaped" curve when they are laid flat, as in a dryer, thereby exacerbating the problems of web weave and shift. Web shifting and weaving may also result when the web tension is low, and when a lighter weight web, such as polyester film, is used.
When the travelling web exits the drying apparatus it is generally wrapped around one or more rotating take-up members, such as chill rolls. When web weave or shift takes place as the web travels through the dryer, the web will correspondingly shift or weave as it contacts the rotating take-up member. Unless the web can be brought back to a straight orientation with respect to the centerline, the web will not wrap properly and the press must be shut down. This results in costly downtime and waste.
Other problems may result from web weave or shift, e.g. if the web moves to one side or the other and stays in the new position it may not be centered on the press, resulting in an unacceptable product; if the web weave or shift is severe enough the web may break or tear, etc. These problems also force a shut-down of the press and thus a loss of valuable production time.
In the past, attempts have been made to circumvent web weave and shift by guiding the web using contact systems such as a series of rollers. These systems are undesirable as they may cause damage to the ink or coating on the web, and are therefore inefficient in situations where there is a high length to width ratio.
Thus it is desired to provide a means by which a web may be floatingly supported and urged in a substantially straight path at points in the dryer where it has a tendency to weave or shift as it travels through a web drying apparatus.